Communication systems allow users to communicate with each other over a network. The network may be, for example, the Internet or public switched telephone network (PSTN). Audio signals can be transmitted between nodes of the network, to thereby allow users to transmit and receive audio data (such as speech data) to each other in a communication session over the communication system.
A user device may have an audio output device such a speaker or set of speakers for outputting audio signals to near end user. The user may enter into a communication session with another user, such as a private call (with just two users in the call) or a conference call (with more than two users in the call). The audio signals may be received over the network from a far end user during a call. The user device may also have audio an input device such as a microphone or array of microphones that can be used to receive audio signals such as speech from a user. The user's speech is received at the microphone, processed and is then transmitted over a network to the other users in the call.
As well as the audio signals from the user, the microphone may also receive other audio signals, such as background noise and echo, which are unwanted and which may disturb the audio signals received from the user. For example, in a call, the near end user's microphone signal received at the far end device via the network may be outputted via the far end user's loudspeakers. This is turn may be picked up by the far end microphone, and transmitted back to the near end device, so that the near end user's own microphone signal is played out of their loudspeakers. This is an example of an acoustic loop, which can lead to acoustic feedback when the system gain is high. Acoustic loops, whereby a microphone signal is outputted by a loudspeaker in the vicinity of the microphone itself, and received by the microphone, can arise in other contexts, such as an acoustic system with a single audio device. That is, other types of acoustic system are prone to acoustic feedback also.